Before adulthood, the brain naturally produces significant amounts of nerve cells called neurons, which power the main coordination and control functions of the central nervous system.
Doctors and scientists refer to the process of neuron production as neurogenesis. During adulthood, neurogenesis essentially comes to a halt in almost all areas of the brain except for a region called the hippocampus, which plays a primary role in certain functions related to memory and learning.
According to findings reported in 2011 by the National Institute on Drug Abuse, use/abuse of the illegal stimulants cocaine and methamphetamine can interfere with adult neuron growth inside the hippocampus. In turn, disruption of neuron production in this brain region can increase overall risks for the onset of a stimulant addiction.
The purpose of neurogenesis is to populate or re-populate the brain with neurons. These nerve cells form the core of the communications network that makes the brain the center of all nervous system activity within the rest of the body. The primary period of neuron production occurs before birth during the process of fetal growth and development. However, neurogenesis also continues during childhood and adolescence in a less extensive form. By the time an individual reaches adulthood, the production of new neurons comes to a halt except inside the hippocampus and in a region called the subventricular zone, which sits near fluid-filled chambers in the brain known as the lateral ventricles.
The hippocampus is a paired structure that roughly resembles two small bananas or horns. Along with several other structures in the brain, it belongs to a circuit or pathway known as the limbic system. As a whole, the limbic system plays a primary role in the human ability to experience rewarding or pleasurable sensations. However, as a distinct structure, the hippocampus functions largely as the brain’s center for taking in new memories, arranging those memories and creating an index that makes the newly arranged memories easy to find in the future. Other vital tasks carried out by this brain region include coordinating emotion-based responses to internal or external situations, helping the body orient itself spatially to its surroundings, and providing the ability to remember and navigate the details of previously encountered physical environments.
Effects of stimulant abuse
Like virtually all other substances of abuse, cocaine and methamphetamine produce their primary effects inside the brain by boosting the presence of dopamine, a neurotransmitting chemical that activates the pleasure-producing neurons contained within the limbic system. As noted previously, the limbic system includes the hippocampus, along with several other brain structures. According to the results of two separate studies published in 2008 in the Journal of Neuroscience and Biological Psychiatry, the presence of either cocaine or methamphetamine alters normal adult neurogenesis inside the hippocampus and damages this region’s ability replenish its neuron supply. In particular, both of these stimulants interfere with the basic processes of neurogenesis at an early, critical stage known as proliferation. In addition, cocaine interferes with the ability of existing immature neurons to grow into their normal mature forms.
Impact on addiction risks
In 2011, researchers from the University of Texas Southwestern Medical Center released the results of a study that examined the addiction risks related to the disruption of normal neurogenesis inside the hippocampus. During this study, adult rat were deprived of their normal ability to produce new hippocampal neurons. These rats were then given free access to cocaine for four hours a day. When compared to adults rats not robbed of their ability to produce new neurons in the hippocampus, these neurogenesis-impaired rats consistently increased their cocaine intake by 60%. The neurogenesis-impaired rats also had a much greater tendency to seek out cocaine between the sanctioned periods of drug access; substance abuse specialists typically view this type of drug-seeking behavior as a critical indicator of a developing drug addiction.
The National Institute on Drug Abuse reports University of Texas researchers also measured neurogenesis-impaired rats’ susceptibility to a drug relapse. During the first phase of this stage of testing, they removed the rats from the cages where drug use had taken place, blocked all drug access for a month, and let the rats go through withdrawal. At the end of the month, the rats were returned to their old cages but still received no access to cocaine. Compared to rats with normal rates of neurogenesis which also went through the same process, the neurogenesis-impaired rats showed more prominent signs of drug craving and a desire to continue drug use (i.e., relapse).
While the studies referenced here implicate cocaine and methamphetamine in particular and stimulants in general, essentially all common substances of abuse damage adult neurogenesis inside the hippocampus. Because of the general effects involved, the researchers from the University of Texas Southwestern Medical Center believe that interference with neurogenesis likely stems from an underlying mechanism common to all of these substances. One potential candidate for this broad mechanism is an alteration of the protein production required to make neurogenesis a possibility.